262 of two to three trained personnel or citizen scientists. Train- ing included an emphasis on the safety of the data collec- tors as well as avoiding any activities that would interrupt the storm recovery crews. Data collection forms had been made available to all trained personnel as part of the train- ing workshops as well as made available online. They included the following: date of loading event, location (community, park name, street addresses), tree metrics (tree species, dbh), damage metrics (type of failure and location of failure, size of branch or trunk at failure), site location metrics (park, lawn, boulevard, right of way, side- walk tree, dimensions of growing space if confined), pre-existing conditions (included bark, codominance, decay, cavities, dead wood, stem-girdling roots, lack of trunk flare at ground line), and a column for any notes not previously addressed in the spread sheet. Since the data were largely being collected while emer- gency recovery activities were in process, not every tree was assessed immediately following the wind loading event. In those cases, damaged and undamaged trees in high risk areas were revisited after emergency operations had concluded and data were recorded at that time. All data collectors were instructed to not interpret or project, but to treat each tree as a snapshot. If there were any questions on tree identification or pre-existing conditions, they were instructed to take photographs and forward them to the principle investigator. All surveys submitted included the name(s) of the trained personnel or citizen scientists. Upon completion of the data collection, a tree species survey was conducted within the areas where the data were collected to add an element of perspective to the numbers and spe- cies suffering damage from the wind loading event. All data were reviewed and entered on a master spread sheet for each wind loading event and dated. At the conclusion of the 11-year study, the data were analyzed by the princi- ple investigator. Wind Loading Event in Minneapolis, Minnesota, June 21, 2013 Background During the early evening hours of June 21, 2013, a violent thunderstorm swept through much of metropolitan Minne- apolis/Saint Paul, Minnesota, wreaking significant damage to trees, utilities, and structures. This storm was character- ized by two unusual phenomena. Earlier in the same day, a Beaufort Wind Scale force number 9 thunderstorm moved from the southwestern part of the state and through the metropolitan area, delivering rain and winds ranging from 45 to 50 miles per hour (mph)(National Weather Service Forecast Office [NWS], Chanhassen, MN, 6/21/13). Sev- eral hours after this storm caused minor damage to trees and infrastructure, a second wind storm with maximum wind speeds recorded at 61 mph (Beaufort Wind Scale ©2019 International Society of Arboriculture Johnson et al.: Boulevard Tree Failures force number 10) followed, accompanied by torrential rains of up to 2.5 inches in less than two hours (NWS, 6/21/13). Even though the earlier storms originated in the southwestern portion of the state, as the storms moved north and eastward toward Minneapolis, the path of both wind loading events altered their original paths and extended from the northwest corner (290°, NWS, 6/21/13) of Minneapolis and moved diagonally to the southeast cor- ner. The neighborhoods that were hardest hit were those in the southern half of the city. Minneapolis lost many mature trees, including hundreds in the boulevards and parks, as well as a significant amount of structural damage to stand- ing trees in public and residential landscapes. The Minneapolis Park and Recreation Board (MPRB) Division of Forestry contracted with the University of Minnesota Department of Forest Resources Urban For- estry Outreach Research and Extension laboratory to con- duct a detailed survey of tipped (aka, wind thrown or whole tree loss) or partially tipped trees, any associated pre- existing conditions that the trees were subjected to, and determine if there were any significant relationships between the noted variables (cause) and the tipped or par- tially tipped trees (effect). As per the agreement with MPRB, the stumps and root systems of affected trees were left in place until all tree failure data could be collected. All data collection began in August of 2013 and concluded by the end of December, which included the collection of information on sidewalk and street improvement activities (secondary data) with the cooperation of the Minneapolis Public Works Department. The University of Minnesota’s School of Statistics was contracted by the Urban Forestry Outreach Research and Extension lab to design the research sampling protocol and conduct the final data analysis. To that end, a study proto- col was developed that included sampling both tipped and undamaged trees along the path of the storm. This path was a stratified path through the city boundaries that included block street segments (BSS) that contained at least two tree failures. A BSS was defined as both sides of a street that divided two paired blocks and included all of the trees in the boulevards on both sides of the street, damaged or not. Streets were selected that ran approximately north/south to roughly parallel with the path of the storms which would also experience the greatest force of the prevailing winds throughout the wind loading event. To establish the number of BSS that would be included in the survey, a rapid visual assessment of tipped or par- tially tipped boulevard trees was conducted and mapped with GPS coordinates and street addresses (Figure 2). Con- sistent with the recorded path of the wind loading event, the majority of these trees occurred in a northwest to south- east pattern, with the greatest number of impacted trees in the southeast corner. Tipped trees were defined as those
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